Preparation of phosphazene rubbers normally involves preparation and purification of dichlorophosphazene (NPCl.sub.2).sub.n oligomers, e.g. wherein n ranges from 3 to 9, polymerization of the oligomer to a higher linear polymer (e.g. with a degree of polymerization ranging from about 20 to about 50,000) which is denoted chloropolymer, purification of the chloropolymer, and reaction of the purified chloropolymer to impart desirable properties.
In one type of reaction of chloropolymer to impart desirable properties, said chloropolymer is reacted with alkali metal alkoxide which may be fluorine substituted to replace chlorine in the polymer with alkoxy and/or fluoroalkoxy. The term alkoxyphosphazene polymer is used herein to denote the polymer reaction product regardless of whether its substituents are alkoxy or fluoroalkoxy or both. A by-product of the reaction is alkali metal chloride. This reaction is well known. See, for example, Shibuta et al. U.S. Pat. No. 4,446,295; Antkowiak et al. U.S. Pat. No. 4,357,458; Allcock et al. U.S. Pat. No. 4,237,263; Fieldhouse et al. U.S. Pat. No. 4,129,529; Fieldhouse et al. U.S. Pat. No. 4,128,710 and Rose U.S. Pat. No. 3,515,688.
It is desirable to carry out the chlorine replacement reaction in a homogeneous system, i.e. using solvents such that the polymer reaction product remains dissolved in the reaction solvent(s). Use of such a homogeneous system improves the reaction kinetics of the chloride replacement reaction. This concept is known from Allcock et al. U.S. Pat. No. 4,237,263 and Fieldhouse et al. U.S. Pat. Nos. 4,129,529 and 4,128,710 where tetrahydrofuran is used as the sole solvent.
It is also possible to obtain a homogeneous system using a combination of solvents for the reactants, i.e. tetrahydrofuran as a solvent for the alkoxide reactant and also either cyclohexane or cyclohexane plus C.sub.5 -C.sub.7 alkane as a solvent for the chloropolymer reactant where the volume ratio of tetrahydrofuran to the other solvents exceeds about 1.00:1. This allows the chloropolymer to be dissolved in cyclohexane or in cyclohexane plus C.sub.5 -C.sub.7 alkane for admixture with alkoxide dissolved in tetrahydrofuran for reaction. This is advantageous over admixing the chloropolymer dissolved in tetrahydrofuran because there is a potential reaction between the chloropolymer and tetrahydrofuran.
This homogeneous system which relies on tetrahydrofuran and cyclohexane and optionally on C.sub.5 -C.sub.7 alkane undergoes reaction to provide a reaction solution wherein the formed alkoxyphosphazene polymer is in solution in the tetrahydrofuran/cyclohexane/optionally alkane, and the alkali metal chloride is colloidally dispersed in the solution. This reaction solution is purified by desalting and then removing the solvents. The desalting is readily carried out by treating with water plus surfactant. The solvent removal presents a problem if it is to be achieved economically. Evaporation is costly and tetrahydrofuran evaporates first so that the polymer coagulates and can trap some of said solvents. Coagulation by adding a non-solvent liquid avoids this problem.